The stable fluidic dispersions of magnetic nanoparticles especially, ferrofluids have been conscientiously investigated in recent years for their numerous potential applications. Their remarkable magnetoviscous effects have made them highly desirable for scientific and engineering applications e.g. vacuum seals (Journal of Magnetism and Magnetic Materials, 1990, v. 85, no. 1-3, p. 233-245), microelectromechanical systems (MEMS) (Sensors and Actuators A: Physical, 2000, v. 84, no. 1, p. 176-180), magnetic hyperthermia (Journal of physics D: Applied physics, 2003, v. 36, no. 13, p. R167), hearing aids (Indian Journal of Engineering and Materials Sciences, 2004, v. 11, p. 241-252), contrast enhancement in MRI (Journal of Magnetism and Magnetic Materials, 2005, v. 289, p. 328-330), drug delivery and its targeting (Drug Development Research, 2006, v. 67, no. 1, p. 55-60; Nano Today, 2007, v. 2, no. 3, p. 22-32) etc. have been explored. On the other hand, since the historic discovery of carbon nanotubes (CNTs) by Ijima (Nature, 1991, v. 354, p. 56-58), there has been great interests towards its unique electrical, thermal, optical and mechanical properties (Nature, 1996, v. 382, p. 54-56.; Physical Review B, 1998, v. 68, p. 6756-6759; Physical Review B, 1999, v. 59, no. 4, p. 82514; Synthetic Metals, 1999, v. 103, p. 2555-2558; Physical Review Letters, 2000, v. 84, no. 20, p. 4613). Instead, a fewer attempts have been given to enhance their magnetic properties (The Journal of Physical Chemistry B, 2005b, v. 109, no. 41, p. 19060-19063; Nano Letters, 2005, v. 5, no. 5, p. 879-884; Nanotechnology, 2009, v. 20, no. 5, p. 055607; The Journal of Physical Chemistry C, 2010, v. 114, no. 15, p. 6944-6951, Kim, I. T., and R. Tannenbaum, 2011, www.intechopen.com; Magnetic Carbon Nanotubes: Synthesis, Characterization, and Anisotropic Electrical Properties; Carbon, 2011, v. 49, no. 1, p. 54-61).
In U.S. Pat. No. 6,975,063, methods are presented for the metallization of carbon nanotubes for field emission applications. The metallization procedure involves an electrodless plating technique for depositing thin metal layers of few nanometers on SWCNT, MWCNT, etc. Thereafter, dispensing the dispersed mixtures of metallized carbon nanotubes onto any suitable substrate for a thin coating of this material. The presence of magnetic field initiates the alignment of magnetically-active metallized carbon nanotubes. The present invention also directs method for cold cathode field emitting materials comprising metallized carbon nanotubes and using metallized carbon nanotubes as cold cathode field emitters.
In U.S. Pat. No. 8,038,926, the carbon nanotubes with embedded gold, SiC and Fe3O4 nanoparticles are produced. Initially a template of anodized porous aluminum oxide membrane (pore diameter of 20-200 nm) is placed in a solution of desired nanoparticles followed by sonication. The template is separated by treatment to a basic solution and a carbon structure via chemical deposition of carbon on the template is formed.
In U.S. Pat. No. 8,119,021, the nanotubes prepared via chemical vapor deposition on alumina template were loaded with functionally active gold and Fe3O4 nanoparticles. The application of magnetic field therein helps in manipulation of CNTs encapsulated with magnetic particles, drugs or quantum dots.
In U.S. Pat. No. 8,325,079, a radar absorbing (0.1-60 MHz) composite material includes a (CNT)-infused fiber material. The material exhibits radar reflectance as well as dissipation of radar energy in a systematic layer by layer coated form.
The general drawbacks of hitherto known ferrofluid-CNT composites is that they are all in solid state. The manipulation of such homogenized composite in a well dispersed liquid state can give more flexibility in its magnetic and electrical response as mentioned below.
In U.S. Pat. No. 6,975,063, inventors have utilized the carbon nanotubes metallized with magnetically active, in thin coated form for cathode field emitters. A thin metallic coating induces the magnetic response in the prepared composite.
In U.S. Pat. No. 8,038,926, a composite material comprising carbon nanotubes with embedded with magnetic nanoparticles, is prepared in dried state. The size selection of nanoparticles ranges from 20-200 nm, depending on porosity of template.
In U.S. Pat. No. 8,119,021, empty carbon nanotubes have been loaded with functionalized magnetic nanoparticles for carrying out desired magnetic manipulation.
In U.S. Pat. No. 8,325,079, a CNT-infused fiber material is used in coated form for radar absorbing applications.
Our invention provides a new method to prepare a stable new hybrid nanofluid comprising a composite of ferrofluid and functionalized MWCNT. In each of these earlier inventions, the preparation of this magnetic composite has been described in dried state. This limits the control orientation of CNTs loaded with nano-magnetic particles. A homogeneous distribution of CNTs is not achievable in dried state. Further, the uniform coating of CNTs loaded with magnetic particles is not attainable in dried state. None of the inventors has prepared the ferrofluid-MWCNT composite in liquid state. The present method is successful in synthesis of this composite and its stable dispersion in water. Therefore, the preparation method resulting in aqueous dispersion of ferrofluid-MWCNT composite is unique and opens door for several other possible applications.